Another chemical commonly added to soap is _Borax_. In view of its alkaline reaction to litmus, turning red litmus blue, this salt is no doubt generally regarded as alkaline, and, as such, without action on soap. On the contrary, however, it is an acid salt containing an excess of boric acid over the soda present, hence when it is added to soap, fatty acids are necessarily liberated, causing the soap to quickly become rancid. As a remedy for this it has been proposed to add sufficient alkali to convert the borax into neutral mono-borate of soda which is then added to the soap. This process is patented and the name "Kastilis" has been given to the neutral salt. The incorporation of borax with the addition of gum tragasol forms the subject of two patents (Eng. Pats. 4,415, 1904; and 25,425, 1905); increased detergent and lasting properties are claimed for the soap. Another patented process (Eng. Pat. 17,218, 1904) consists of coating the borax with a protective layer of fat or wax before adding to the soap with the idea that reaction will not take place until required. _Boric acid_ possesses the defects of borax in a greater degree, and would, of course, simply form sodium borate with liberation of fatty acids, so should never be added to a neutral soap. _Salicylic Acid_ is often recommended for certain skin diseases, and here again the addition of the acid to soap under ordinary conditions results in the formation of sodium salicylate and free fatty acids. To overcome this a process has recently been patented for rubbing the acid up with vaseline before addition to soap, but the simplest way appears to be to add the soda salt of the acid to soap. Amongst the more common milled medicated toilet soaps may be mentioned, in addition to the above:-- _Birch Tar Soap_, containing 5 or 10 per cent. birch tar, which has a characteristic pungent odour and is recommended as a remedy for eczema and psoriasis. _Carbolic Soap._--A toilet soap should not contain more than 3 per cent. of pure phenol, for with larger quantities irritation is likely to be experienced by susceptible skins. _Coal Tar._--These soaps contain, in addition to carbolic acid and its homologues, naphthalene and other hydrocarbons derived from coal, naphthol, bases, etc. Various blends of different fractions of coal tar are used, but the most valuable constituents from a disinfectant point of view are undoubtedly the phenols, or tar acids, though in this case as with carbolic and cresylic soaps, the amount of phenols should not exceed 3 per cent. in a toilet soap. An excess of naphthalene should also be avoided, since, on account of its strong odour, soaps containing much of it are unpopular. The odour of coal tar is considerably modified by and blends well with a perfume containing oils of cassia, lavender, spike, and red thyme.